This invention relates to electron discharge tube cooling systems, and more specifically to liquid evaporation cooling systems for klystrons and other high-power electron discharge tubes.
In such tubes, a major proportion of the heat to be removed by the cooling system may be generated in one specific part of the tube, for example in the collector of a high-power klystron.
A liquid cooling system for such tubes is described in United Kingdom Patent Specification No. 1,114,513. In this system, a part of a tube to be cooled is immersed in coolant in a boiler. The coolant is supplied to the boiler by pump means for a coolant reservoir via an inlet pipe at a low level therein. The coolant is also supplied to an overflow system which is external to the boiler and has an overflow level at the level desired to be maintained in the boiler, coolant overflowing in the overflow system being returned to the reservoir. Vapourised coolant from the boiler is condensed by a condenser and the condensate is returned to the coolant reservoir. An additional cooling chamber, adapted to cool a separate part of the electron discharge tube, such as the drift tube assembly of a klystron, may be connected in series with the coolant flow to the boiler and overflow system.
In arrangements of the type described in U.K. Specification No. 1,114,513, the overflow system is subject to atmospheric pressure. If the pressure in the boiler exceeds atmospheric pressure, the excess pressure will depress the level of the coolant in the boiler below the desired level.
Although the condenser for the vapour generated in the boiler may itself be vented to atmosphere, the boiler will still be subject to a back pressure caused by the resistance which the flow of vapour experiences in the interconnected pipework from the boiler to the condenser. It is therefore necessary to arrange the interconnecting pipework so that the back pressure does not depress the coolant in the boiler below a safe level. The tube manufacturer may specify a maximum value, typically 2.5.times.10.sup.2 Pa (2.5 cm water gauge), for the back pressure in order to ensure safe operation of the tube in a cooling system of the type hereinbefore described.
In many applications the requirement that the back pressure shall not exceed a low value such as 2.5.times.10.sup.2 Pa is difficult to obtain and requires expensive measures. In the case of a kylstron rated t 45 kW output power such as may be employed in an output stage of a television transmitter, it is focused necessary to use pipes of 4 inch (10 cm) internal diameter between the boiler and the condenser even when the latter can be located close to the former. In many instances, considerations of available space make it necessary to position the condenser at some distance from the boiler. In such instances, pipes of 6 inch or 8 inch (15 cm or 20 cm) internal diameter may be necessary to avoid excessive back pressure. Moreover, to avoid contamination of the coolant, which typically comprises deionised water, the pipes must be fabricated from materials such as high-purity copper or suitable grades of stainless steel. Such pipework is difficult to fabricate and to install, and is extremely costly. Substantial savings in pipework costs would be possible if the desired coolant level in the boiler could be obtained for a range of back pressures.